This invention relates to the non-invasive measurement of the concentration of substances that absorb electromagnetic radiation, such as light or infrared radiation, in absorbing and turbid matrices, such as human or animal body tissue, using a probe beam of electromagnetic radiation. The invention is described as applied to the special case of glucose measurement in human tissue using near-infrared radiation. It is, however, generally applicable to measurements of the concentration of any species that absorbs electromagnetic radiation, especially in strongly absorbing and turbid matrices.
The infrared measurement methods known in the art are not well adapted to the problem of quantifying an analyte dissolved in a strongly absorbing solvent. The known methods include separate or directly alternating measurements at a "glucose" wavelength and at a "reference" wavelength, where glucose does not absorb, as well as differential wavelength modulation about a glucose absorption band (C. Dahne, D. Gross, European Patent 0 160 768 and references therein). In the known methods, the signal is easily lost into the variable and strong background presented by water and other constituents in the tissue and in the capillary blood flow.